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Diagnosis means the identification of the nature and cause of an illness. The word diagnose means to determine what disorder or disorders one may have, and diagnosis is used to mean the specific name of the disorders, such as major depressive disorder. The plural of diagnosis is diagnoses, meaning more than one disorder. In all areas of medicine, diagnoses are based upon three general types of information.

The first of these is the medical history. It is a patient’s report of his or her illness. For example, “I have been feeling depressed almost every day for about three months.” The second is the physical exam, which refers to the hands-on examination of the patient by the doctor or other health care professional. The final category is tests. These can include lab work, such as a complete blood count or thyroid function tests, X-rays or more invasive procedures, such as a biopsy.

How Depression is Diagnosed

Depression is diagnosed almost exclusively on the history (report of symptoms experienced). This is because there are typically not physical signs that can be detected by physical exam, nor are there laboratory abnormalities. Observations by the clinician are also factored into the diagnosis. In the case of minors, reports made by parents and teachers are also typically included in the process of diagnosing. A physical exam and laboratory tests can also be useful to help rule out medical causes of symptoms.

The reason there are no objective laboratory tests for depression has to do with the complexity of the human brain. Our brains are so complex that it is very challenging to understand human brain function. It is even more difficult to understand many brain disorders, such as depression.

Diagnostic Criteria

The specific diagnostic criteria for a major depressive disorder are listed in the Signs and Symptoms of Depression section. This section provides some useful background information.

The diagnostic criteria for depression and all psychiatric disorders are defined in the Diagnostic and Statistical Manual of Mental Disorders (DSM). The current version is the fifth revision, so it is referred to as the DSM 5. The American Psychiatric Association, the national organization for physicians specializing in psychiatry, publishes the DSM. It outlines the diagnostic criteria for all psychiatric disorders and provides supplementary information about disorders, such as inheritability, other disorders that may be similar, prevalence and course of illness. Prominent psychiatrists and other mental-health professionals write the DSM based upon research suggesting how psychiatric disorders should be defined. It is updated periodically as new information becomes available.

Limitations of Diagnosing Depression

A limitation of DSM is that it is not based on the biological causes of disorders. In medicine, almost all disorders are defined, at least in part, by the underlying pathology or what is physically wrong in the body. We do not yet understand the neurobiology of psychiatric conditions well enough to use pathology as a part of the definition. As science advances, this may be possible in the future. For now, we are limited to definitions based upon the existence of symptoms. The problem with this is that there may be more than one pathological process that can lead to the same set of symptoms. This is thought to be true for depression. There are likely many causes of depression. One cause may respond to a particular treatment while another does not. This is why more than one treatment for depression must be tried in order to find one that works for that particular patient.

Other Causes of Depression

Another disorder, a medication or a substance can sometimes cause depressive symptoms. Some medical conditions that can cause depressive symptoms include Crohn’s disease, diabetes, fibromyalgia, heart disease, hypothyroidism, liver disease, multiple sclerosis rheumatoid arthritis, stroke and traumatic brain injury. A variety of prescribed medications and drugs of abuse can also cause depressive symptoms. Thus, a thorough evaluation by a medical or mental-health professional is required to diagnose depression and rule out other causes of symptoms.

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About the Author

William R. Marchand, M.D., is the Chief of Psychiatry at the George E. Wahlen VAMC in Salt Lake City, Utah, and a Clinical Associate Professor of Psychiatry at the University of Utah. He is the author of "Depression and Bipolar Disorder: Your Guide to Recovery" and "Mindfulness for Bipolar Disorder: How Mindfulness and Neuroscience Can Help You Manage Your Bipolar Symptoms."